The invention concerns a measuring device having at least one detection coil for the detection of nuclear magnetic resonance (NMR) from a sensitivity region of the detection coil within the earth, whereby nuclear spins within the sensitivity region are, in each case, excited to precess in the earth's field through repeated coherent excitation to produce a small NMR induction voltage in the detection coil which is introduced to a first amplifier.
A coil system of this kind is known in the art, for example, from DE 36 90 746C2. In this publication one attempts to extract spatially resolved information from the earth using NMR measurements. Towards this end a coil is placed on the earth's surface to produce an alternating field for the excitation of magnetic resonance in the earth's magnetic field. Subsequently, the resonance signal due to the free precession of the nuclear spins in the earth is measured using this coil.
The spatially resolved extraction of NMR signals from the earth is, in addition, known in the art from EP 0 512 345 A1, whereby, using surface coils, a main magnetic field, magnetic gradient fields, as well as a high frequency field for the excitation of the NMR signals are produced.
The elimination of interferences occurring during the recording of NMR signals in a configuration similar to that of DE 36 90 746 C2is known in the art from U.S. Pat. No. 3,019,383, wherein following each excitation, one records, using two receiving channels which are stored to magnetic tape, the interference together with the usable signal in one channel, and, following a waiting time, the interference only in the other channel. Both channels are simultaneously read-out and effectively subtracted. One collects data over a plurality of excitations to thereby suppress highly distorted signals.
The origin of NMR signals is usually moisture in the earth. The signals are extremely small so that, for a single excitation, they are buried in the noise of the measuring configuration. In a typical environment which can also be outside of a residential area, the received signal is nevertheless dominated by interferences which are generally associated with (but are not completely due to) the power line frequency of 50 Hz or the railway frequency of 16 2/3 Hz and their higher harmonics. Since there is no fixed phase relationship between the NMR excitation and the interfering signals, the interferences as well as the noise should be slowly averaged-out through accumulation of a large number of signals. For interferences which do not change within the framework of the repetition time of the excitation it is possible to utilize a subtraction procedure as in the above-mentioned U.S. Pat. No. 3,019,383. Towards this end, however, there is a dynamic range problem, since the largest interfering signal limits the maximum amplification.
However, it has been surprisingly demonstrated in field trials that although spatially homogeneous disturbances constitute the largest fraction of the interfering signals, a sizable portion is nevertheless due to inhomogeneous local interferences which are largely in the frequency range of the power lines or the railway frequency and their upper harmonics without being proximate to underground power lines or elevated power lines which could have explained same. The magnetic field lines of this interfering portion are not parallel and equidistant over the area of the measuring coil configuration but rather are diagonal thereto and the field strength has a spatial dependence.
Since the NMR measuring coil configuration is normally horizontally located on the ground, it can only detect signal components whose magnetic field vector is perpendicular to the earth's surface (z-direction). The inhomogeneous interfering fields, however, change from one partial detection coil to another with respect to their field strength as well as as their direction and are therefore not compensated for or not fully compensated for using a configuration having at least two coils which, in particular, are located in a common plane.
It it therefore the purpose of the present invention to improve a coil system of the above-mentioned kind in such a fashion that the interfering signals which occur are more satisfactorily eliminated.